Self-moderated just in case.
While the block reward stills gives a fair amount of bitcoins, it won't do in one or two decades. The miners will have to arrange their business based on the transaction fees. And now I'm asking; how will the system work if we all move to the LN? Sure, by opening and closing channels we broadcast transactions to the main layer, but if the majority behaved just like how I did, the mempool won't ever full. If it doesn't full, there will be less competition of who will get their transaction confirmed first and thus, less fee. I'm afraid that, overtime, there will be less incentive which is also translated to less security.
There will always be a block reward, until the very last bitcoin is mined. The Mempool will remain full, assuming the scale of the LN reaches certain levels. The sheer amount of channels being opened and closed will surely keep the Mempool quite active. Not to mention, on-chain transactions will always have their place.
On the other hand, I am curious to how miners will react after a few more halving when the block reward is smaller. Then again, the value of bitcoin may be much larger, thus incentivizing still.